This invention relates generally to a valve assembly for controlling the outlet from a liquid dispenser, and more particularly to such an assembly which is capable of effecting a suck-back flow of liquid from its nozzle after closing the discharge passage.
Valve or nozzle assemblies of the general class herein, having a suck-back valve or draw-back capability after closing the discharge, include some type of discharge valving, and a chamber downstream thereof in communication with the nozzle for effecting a suck-back flow of liquid from the nozzle upon an increase in volume in the chamber. The simplest approach includes the use of a hand-operated plunger which forces the liquid, such as an adhesive, out of a discharge nozzle from an adjoining chamber containing the liquid, and subsequent retraction of the plunger draws-back product from the nozzle to prevent product extrusion from the nozzle tip after a predetermined amount is dispensed or applied to a workpiece. This has proven largely ineffective for accurately cutting of the flow of product through the nozzle for liquids of different viscosities, and is obviously unsuitable for large scale dispensing operations. Other valve arrangements provide for a more precise cutting off of the flow of product from the nozzle after a closing of the discharge valve. However, many of such arrangements require relatively movable parts to effect a suck-back flow from the nozzle, or separate elements for closing the discharge and thereafter creating the suck-back flow. Otherwise, external means are required for carrying out the discharge closing and/or draw-back operations. These various prior art approaches thus require numerous parts and external functions which add to the complexity and cost of the valve arrangement and are more cumbersome to operate.